As semiconductor devices including integrated circuits (IC) operate at higher frequencies, higher data rates and lower voltages, the need to reduce noise in the power and ground (return) lines, the need to supply sufficient current to maintain the power level, and the need to accommodate faster circuit switching become increasingly important issues. These needs require low impedance in the power distribution system. In order to reduce noise and provide stable power to the IC, impedance in conventional circuits is reduced by the use of additional surface mount technology (SMT) capacitors interconnected in parallel. The higher operating frequencies (higher IC switching speeds) and shorter rise times mean that response times of the power delivery network supplying power to the IC must be faster. Lower operating voltages require that allowable voltage variations (ripple) and noise become smaller. For example, as a microprocessor IC switches and begins an operation, it calls for power to support the switching circuits. If the response time of the voltage supply is too slow, compared to the rise time of the signal, the microprocessor will experience a voltage drop or power droop that will exceed the allowable ripple voltage and noise margin and the IC will malfunction. Additionally, as the IC powers up, a slow response time will result in power overshoot. Power droop and overshoot must be controlled within allowable limits by the use of capacitors that are close enough to the IC that they provide or bypass power within the appropriate response time.
For ICs mounted on the surface of a printed wiring mother board, SMT capacitors for impedance reduction and dampening power droop or overshoot are generally placed on the surface of the board as close to the IC as possible to improve circuit performance. Conventional designs have capacitors surface mounted on a printed wiring board (PWB) clustered around the IC. Large value capacitors are placed near the power supply, mid-range value capacitors at locations between the IC and the power supply and small value capacitors very near the IC.
High power and high frequency ICs are generally mounted on a semiconductor package. The semiconductor package is generally only somewhat larger than the IC or ICs. The semiconductor package, complete with mounted ICs, is conventionally mounted to a larger printed wiring mother board or daughter card. In this situation, large and medium value capacitors may reside on the printed wiring mother board or daughter card to which semiconductor package is attached. There is, however, a limitation to the number of SMT chip capacitors that can be mounted in parallel on a semiconductor package.
As IC frequencies increase and operating voltages continue to drop, increased power must be supplied at faster rates requiring increasingly lower impedance levels. Impedance decreases with decreasing inductance and increasing capacitance. Hence it is necessary to minimize the inductance of the interconnection between the capacitors and the IC.
U.S. Pat. No. 6,611,419 to Chakravorty discloses that power supply terminals of an integrated circuit die can be coupled to the respective terminals of at least one embedded capacitor in a multilayer ceramic substrate. U.S. Pat. No. 7,029,971 discloses thin film dielectric for capacitors fired on metallic foils for incorporation into printed wiring boards and the problems with oxidation that arise when high dielectric constant dielectrics are fired on copper foils at elevated temperatures. U.S. Patent Application US20080316723A1 to Borland et al. discloses methods for incorporating high capacitance thin-film capacitors into the build-up layers of a printed wiring board, such as a semiconductor package. The thin film capacitors may be formed on copper or nickel foils.
Forming ceramic capacitors on a nickel foil has the advantage that nickel is much more resistant to oxidation during heated dielectric prefiring steps and during dielectric firing. However, nickel foils do not adhere well to photoresists or organic build up materials used in making semiconductor packages. Nickel foils also have poor high frequency signal propagation properties compared to copper foils. Thin nickel foils are also difficult to handle during processing, but when thicker nickel foils are used, it becomes difficult to accurately laser drill interconnection vias in the capacitors being embedded in a semiconductor package. Finally, when one of the metal layers of a thin-film capacitor is a nickel foil and another layer copper, processing and delamination problems result.
Accordingly, improved methods of incorporating a thin-film capacitor into a semiconductor package wherein the thin-film capacitor is formed on nickel foil are needed.